Added SKS Keyserver role

sks is a OpenPGP Keyserver with automatic synchronization between
different instances.

By default, sks will be configured but not started, because it won't
start without a database. If you want to start your keyserver with empty
database, you can set 'sks_autoinit: True' in your Ansible inventory.

If you want to connect to or use public PGP/GPG Web of Trust, it's
strongly recommended to download the key database dump and build local
key database by hand. More information can be found in sks
documentation, and on http://keysigning.org/sks/ website.